1. Field of the Invention
This invention relates to transmissions used typically in multi-wheeled cycles such as bicycles, and more particularly to mechanisms that allow for smooth shifting of such transmissions between gears of varying ratio.
2. Description of Related Technology
Multi-geared bicycle transmissions have been known for many years. Most bicycles achieve multiple gear ratios by utilizing an arrangement of sprockets on a front sprocket assembly, commonly called the chain ring or chainwheel, and a rear sprocket assembly, called the freewheel or cassette. Typically, the chainwheel comprises two to three sprockets of varying diameter. The freewheel assembly typically comprises five to seven sprockets, also of varying diameter, but each smaller than the diameter of the sprockets on the chainwheel. More recently, freewheels with as many as 9 sprockets have been introduced. A drive chain coupling the chainwheel and the freewheel make up the drive train. As the chain is coupled to different sprockets on both the chainwheel and freewheel assemblies, a different effective drive ratio of the transmission is obtained. Each combination of chainwheel/freewheel sprockets in use by the drive chain represents a different gear ratio, and a different gear. Obviously, the more sprockets on both the chainwheel and the freewheel, the greater the number of available gear ratios, and, consequently, the greater number of gears.
For almost as many years, manufacturers and cyclists alike have recognized the need for efficient, smooth shifting from one gear to another, all the while maintaining a positive drive connection during the shifting process. The standard front and rear derailleur, which comprise the shift mechanism of choice for the vast majority of multi-geared bicycles, forces the chain on a holding sprocket toward an adjacent or receiving sprocket until a tooth of the adjacent sprocket catches the chain. The drive chain thus becomes entrained around a different combination of front and/or rear sprockets. Because gears are defined by the ratio of the front holding sprocket to the rear sprocket, the shift from one holding sprocket to an adjacent sprocket of different diameter results in a different gear ratio on the drive chain.
Several systems for effecting such changes of gear ratio are present in the prior art. One such system involves mechanisms for indexing the shift control such that either a front or rear derailleur moves laterally (e.g., transverse or perpendicular to the rotational plane of the sprocket) in response to a predetermined amount of movement of a shift lever, thereby causing the drive chain to move laterally upward or downward among the sprockets in the free wheel cluster (or chainwheel). This type of mechanism is described, for example, in U.S. Pat. No. 3,972,247, "Bicycle Shift Mechanism".
A second prior art system involves a complex lateral displacement of the rear derailleur in relation to the sprockets in a freewheel cluster. In these types of solutions, the rear derailleur starts in a first position in which the drive chain is entrained around one of the cogs in a freewheel and around the wheels of the rear derailleur. Movement of the shift control forces a lateral movement of the rear derailleur to a second position such that the trailing portion of the drive chain that is entrained around the wheels in the rear derailleur becomes aligned with a separate sprocket in the freewheel cluster. This lateral movement, together with a change in the angular position of the rear derailleur with respect to the bicycle frame, forces the drive chain to "jump" from one freewheel sprocket to another.
One embodiment of the aforementioned system is shown in U.S. Pat. No. 5,358,451, "Devices Enabling Shifting of Gears on Bicycles" issued to Lacombe, et al. Lacombe teaches a means of assisting the lateral movement of the rear derailleur along an axle slidably engaged within a housing having multiple control teeth, each tooth being adapted to permit lateral movement corresponding to one shift in the free wheel cluster.
Another embodiment of this type of mechanism is disclosed in U.S. Pat. No. 5,152,720, "Bicycle Transmission" issued to Browning, et al.
However, the foregoing prior art systems fail to address two fundamental shortcomings of modem, multi-geared cycle transmissions; specifically, 1) the high degree of redundancy and overlap between the gears; and 2) the inability to shift gears under a heavy load. To appreciate the significance of these problems, it is necessary fully to understand the operation of the drive train.
The diameter, or size, of the chainwheel and the freewheel sprockets is measured by the number of "teeth" on the sprocket. Chainwheels typically include sprockets having from 36 to 53 teeth. Freewheels include sprockets having from 11 to 34 teeth. Thus, a typical maximum and minimum gear ratio for bicycle drive trains with these sizes of chainwheels and freewheels is 4.82 (e.g., 53/11) for a maximum, and 1.06 (36/34) for a minimum.
Bicycle gearing is typically measured in "gear inches". This concept is derived from early "high-wheeler" bicycles, the gearing for which was measured based on the diameter of their front wheel. A high-wheeler with a 50-inch wheel diameter was said to be geared at 50 gear inches. A bicycle with a 25 inch wheel diameter and a two-to-one gear ratio in the drive train achieves the same forward motion with each pedal revolution as does the 50-inch high-wheeler, and thus is also said to be geared at 50 gear inches. Gear inches are computed by multiplying the drive wheel diameter by the effective gear ratio on the drive train. Using the maximum and minimum drive train gear ratios set out above, and using the normal diameter of a bicycle wheel (27 inches), the minimum gear inches would be approximately 28.6, and the maximum gear inches would be 130.1.
Experienced cyclists are well aware of the significance of gear inches. The greater the number of gear inches (i.e., the "taller" the gear), the more difficult the pedaling due to lower mechanical advantage. Conversely, the lower the number of gear inches (i.e., the "shorter" the gear), the easier it is to pedal. Depending on the gradient of the terrain (and the direction of the cyclist's travel with respect to the gradient), selecting the appropriate gear is not an insignificant task. Pushing a tall gear on the order of 130 inches would be quite difficult going up hills or even on flat terrain, and is consequently usually reserved for pedaling at high rates of speed down steep hills. Pushing a short gear of 28 inches (the so-called "granny" gear) is much easier and is useful in going up steep inclines while carrying baggage on the bicycle. Between these two extremes, there is a virtual continuum wherein a wide selection of gearing is possible. Gear inches of between 30 and 60 are likely adequate to accommodate most hills, reserving the shorter gears for the steeper hills. Gear inches between 60 and 90 are appropriate for travel on level ground, with the taller gears reserved for fast pedaling at high speeds. Gear inches above 90 are appropriate primarily for traveling down hills.
While the frequency and magnitude of gear changes varies in accordance with both the terrain and the degree of rider fitness, most riders prefer a gear change if their pedaling speed, or cadence, increases or decreases by more than ten percent. Gears spaced apart by seven to ten gear inches allow the rider to adjust the gearing up or down, and still maintain a cadence within the desired ten percent variance. Gears spaced closer than seven gear inches produce such a minimal impact on pedaling efficiency that most riders will not notice the difference between adjacent gears. Gears spaced further apart than ten inches will create a larger jump than is comfortable.
To achieve an optimum overall gear range and to provide adequate gear spacing, a common practice in recent years has been to include as many gears as possible on the bicycle in order to accommodate the widest range of terrain. Road bikes (and racing bikes) often have two-ring chainwheels sized at 42 and 53 teeth, and a 8-cog freewheel ranging in size from 13 to 24 teeth. This arrangement will result in the shortest gear being approximately 47 inches, and the tallest gear being 110 inches. Off-road bikes, or mountain bikes, usually have three-ring chainwheels with a typical sizing of 28, 42, and 52 teeth, and a 7-cog freewheel ranging in size from 13 to 34 teeth. This arrangement will result in the shortest gear being approximately 22 inches, and the tallest gear being 108 inches. Obviously, many different combinations are possible, but the foregoing examples are typical.
Because substantial overlap exists in the effective gear ratios of the aforementioned exemplary systems, a sequential "stepping up" through the gears from the shortest gear to the tallest gear is not a simple matter of starting at the shortest gear (achieved by entraining the drive chain around the smallest chainwheel and largest freewheel--42/24 or 47.2 gear inches) and progressing sequentially through the remaining freewheel cogs, and then shifting the drive chain to the larger chainwheel and starting anew at the largest freewheel. Rather, a back and forth shifting between the chainwheels and the freewheels is required, resulting in complicated shift patterns. The shift pattern varies in accordance with the particular type of gearing arrangement.
One such prior art shift pattern is known as the "half-step" gearing. Half-step gearing starts with a freewheel cluster having sprockets spaced at even distances apart. An example of this spacing would be a freewheel selection wherein each successively larger sprocket on the freewheel is about sixteen percent larger than the next closest smaller sprocket. Assuming a five cog freewheel for purposes of illustration, the resulting freewheel would have the following sprockets: 14, 17, 20, 24, and 28 teeth. The half-step approach then selects two chainwheel sprockets that are half as far apart as the sprockets on the freewheel-in this example the spacing would be eight percent. (i.e., 48 and 52 teeth). With this arrangement, the front derailleur shift is half the rear derailleur shift, and results in well spaced and useable effective gear ratios. However, the shift pattern to accomplish a sequential stepping up through the gears in this example is as follows: 48/28; 52/28; 48/24; 52/24; 48/20; 52/20; 48/17; 52/17; 48/14; and 52/14. As can be seen, the half-step pattern requires the rider to shift both the front and rear derailleurs simultaneously to reach four of the ten available gears. Remembering such a shift pattern is a difficult task, and effectuating such shifts consumes more of the rider's attention and time.
Another prior art shift pattern is the so-called "Alpine" pattern which is available on most bikes. The Alpine pattern is essentially the same as the half-step, but uses one and one-half steps. The freewheel spacing of sixteen percent remains the same, but the chainwheel sprockets are selected to be one and one-half times that spacing, or twenty-four percent. This results in two chainwheels of 40 and 52. The Alpine shift pattern suffers from even greater disadvantages than the half-step arrangement, in that 1) simultaneous shifting of both the front and rear derailleurs is necessary for seven of the ten gears; and 2) a large jump exists between the tallest and next-to-tallest gears. Furthermore, the Alpine arrangement involves an more challenging shift pattern, as follows: 40/28; 40/24; 52/28; 40/20; 52/24; 40/17; 52/20; 40/14; 52/17; and 52/14.
A third prior art shift pattern is known as the "crossover". In the crossover arrangement, the freewheel cluster is divided into a first group of closely spaced large freewheel cogs for hill climbing, and a second group of closely spaced small cogs for the flats. Crossover uses the smallest chainwheel in combination with the largest freewheel cogs for hill climbing, then crosses over to the larger chainwheel which is used in combination with the smallest freewheel cogs for the flats and downhills. Small gear steps are used for the flats and larger steps are used for the hills. While this arrangement allows for some gears for hills and some for flats, the division results in fewer gears than would be available with other patterns.
A fourth prior art shift pattern is known as the "wide-step" and is closely related to crossover. Wide-step uses a comparatively large chainwheel step (e.g., 36 to 53) and closely spaced freewheel steps. This permits a low range of gears for hill climbing and a higher range for the flats and downhill. To step up through the gears, a rider would utilize all the gears available with the small chainwheel sprocket, and then switch to the large chainwheel sprocket and repeat use of half of the freewheel sprockets. Effectively, the wide-step pattern creates a bicycle with two independent gear ranges. However, the larger freewheel cogs are not utilized with the larger chain ring.
Regardless of the shift pattern used, one consequence of the multiple chainwheels and freewheels is a high degree of redundancy in the gearing. The chart below illustrates the gear inches that can be achieved by the exemplary prior art road bike previously described. Bicycle A is a 12 speed bicycle which achieves gears of 49.3 inches on the short end, and 110 inches on the tall end. However, two of the twelve gears are within two gears inches of two separate gears, and two other gears are within four gear inches of yet two different gears. Bicycle B is a 16 speed bicycle which achieves a short gear of 47.2 gear inches and a tall gear of 110 gear inches. As was the case with Bicycle A, several of these gears are redundant-four of the gears are spaced within two gears inches of four separate gears. Two other gears are spaced within four gear inches of two separate gears. Thus, while Bicycle A, and Bicycle B have 12 and 16 separate gears, respectively, the practicality of the gear spacing renders four of the gears for Bicycle A, and six of the gears for Bicycle B, redundant of other gears. The end result is an 8-speed Bicycle (Bicycle A) and a 10-speed Bicycle (Bicycle B). [It should be noted that many different chainwheel/freewheel sizes and combinations are possible and result in different levels of redundancy and gear spacing. The following charts simply illustrate two common arrangements.]
Bicycle A Bicycle B 42 53 42 53 13 87.2 110 13 87.2 110 15 75.6 95.4 14 81.0 102.2 17 66.7 84.1 15 75.6 95.4 19 59.6 75.3 16 70.8 89.4 21 54.0 68.1 17 66.7 84.1 23 49.3 62.2 19 59.6 75.3 21 54.0 68.1 24 47.2 59.6
Most manufactures recognize the redundancy in gear ratios, but deem such redundancy necessary to achieve a sufficient number of tall and short gears to meet the expected riding conditions of a typical cyclist.
In addition to gear redundancy, a second factor operates to limit the effective gearing of multi-speed bicycles. A study of the table for Bicycle B presented above shows that the rider could utilize all of the gears in the drive chain comprising the 42 chainwheel and the freewheel cluster to achieve a sequential stepping up through the gears. Crossover to the large chainwheel would not be necessary until after the rider had utilized 42/13 and achieved 87.2 gear inches, at which point, the rider would simultaneously shift both the front and rear derailleurs to 53/15 or 95.4 gear inches and finish stepping up to 53/13.
A smooth transition during the sequential stepping through the gears is not as natural as it would appear. As can be immediately understood by a physical inspection of an ordinary multi-speed bicycle derailleur system, the smaller chainwheel (the 42 chainwheel in the case of Bicycle B above) is positioned on the inside of the larger chainwheel. This positioning aligns the small chainwheel with the inside portion of the freewheel cluster, usually comprising the largest three or four cogs on the freewheel. Accordingly, the small chainwheel is not directly aligned with the outside, smaller cogs on the freewheel cluster. In fact, when the drive chain is entrained around the small (inner) chainwheel and the smaller, outer freewheel clusters, there is a significant lateral or transverse bias present in the drive chain. Referring again to Bicycle B as an example, when the drive chain is entrained around the 42/13, 42/14 and 42/15 gear combinations, the portion of the drive chain which is engaged by the chainwheel is closer to center of the bicycle frame than the portion of the drive chain which is engaged by the freewheel. The resulting lateral bias places undue stress on the chain, reduces pedaling efficiency (due to the force necessary to overcome the additional friction created by the lateral bias), and renders shifting out of one of these gears an often difficult task, particularly when climbing hills, accelerating to high speeds or other situations when the drive chain is under a high load. To avoid the lateral bias in the case of Bicycle B, the rider would need to shift off the small chainwheel after the 42/16 gear and proceed to 53/19. This maneuver would require a simultaneous shifting of both the front and rear derailleurs and would render virtually useless three of the gears--42/13, 42/14 and 42/15.
The same circumstance exists with respect to the large, outside chainwheel and the large, inside cogs on the freewheel. Again using the case of Bicycle B as an example, the gearing represented by 53/19, 53/21 and 53/24 places the drive chain in a lateral bias position in the opposite direction as the previous example (e.g., the freewheel portion of the chain is closer to the lateral frame center than the portion engaging the chainwheel). The difficulty of shifting under load is equally prevalent and, again, three additional gears are virtually useless--53/19, 53/21 and 53/24.
The aforementioned lateral bias problem is another practical reason why many multi-speed bicycles cannot utilize their full range of gearing.
The dual problems of redundancy and lateral bias defy a synergistic solution. Any attempt to solve the redundancy problem using standard components would require resort to a large jump between the sizes of the small and large chainwheels so that the gears could indeed step through short to tall gears by running sequentially through the entire freewheel cluster within each chainwheel. However, this attempted solution merely exacerbates the lateral bias problem by necessitating use of the inside/outside-outside/inside chainwheel/freewheel misalignment. Alternatively, focusing instead on avoiding the lateral bias problem by ignoring the gears where lateral bias is most prevalent requires greater redundancy to achieve an adequate spacing between gears. The net result is that multi-speed bicycles do not effectively utilize their full complement of gears due to high levels of redundancy, and cannot efficiently utilize their full complement of gears due to lateral bias.
Hence, an improved cycle transmission and method of operation are needed which overcome the above-identified disabilities associated with gear ratio redundancy and lateral bias. Such a transmission would also ideally be sized to fit on existing cycle frames and be comparable in weight and size to existing systems.